Icom IC-746 Pro: A current production 100 watt transceiver that
is typical of mid-range ham rigs. It uses digital signal processing
("DSP") for its filtering and other functions, but is not a Software
Defined Radio. It also has the optional TCXO installed.

Flex-Radio SDR-1000: A 100 watt radio that is truly an SDR.
All signal processing is done in a host computer using a sound card for
digital-to-analog and analog-to-digital conversion. The rig comparison
plot shows the SDR-1000's performance using its standard 200 MHz internal
oscillator. The SDR-1000 tests were performed with an M-Audio Delta 44
sound card; I suspect the spurs seen on the SDR-1000 plots come from noise
in the the host computer transmitted via the analog audio cables.

Kenwood TS-520S: A 1970s vintage 100 watt transceiver that uses
hollow state devices (vacuum tubes) for its power amplifier, and has no
synthesizer at all -- it uses a VFO (variable frequency oscillator for you
youngsters) in a traditional superheterodyne configuration.

In each case, I set the rig to generate about 20 watts output in CW mode
(except for the Yaesu FT-817, which was set to 1 watt) and fed the signal
through appropriate attenuators to put about +3dBm into a TSC 5120A phase
noise analyzer. The phase noise reference source was a 5 MHz Wenzel Ultra
Low Noise oscillator. Each transmission was about 5 minutes long, a
compromise to get enough data while not overheating the rig.

If you are unfamiliar with phase noise plots, the Y axis is the power
contained in a 1 Hz bandwidth, referred to the carrier signal level.
The X axis is the frequency offset from the carrier. Note that the X
axis is logarithmic, so close offsets are spread out much more than
distant ones.

The following charts are animations that hold on each frame for 3
seconds... be patient and you'll see the whole thing! (If you'd like to
see the individual images, they are here.

HF Transceivers Compared

SDR-1000 DDS Reference Comparison

I tested the SDR-1000 using its internal 200 MHz oscillator as well as
three external 10 MHz references: a Wenzel ULN oscillator, which
represents the lowest phase noise source you can buy; an HP Z3801A GPS
disciplined oscillator which represents what a "time-nut" ham might use;
and a Marconi 2202A signal generator that represents a worst-case
scenario. (Note for the purists: the Wenzel oscillator runs at 5 MHz and
was doubled to 10 MHz using a Mini-Circuits doubler; the doubler increases
the noise by about 6dB over the 5 MHz input, and its
performance is documented here.)

This is a zoom of the 100 Hz to 100kHz range to show the interesting
behaviour at greater offsets from the carrier (not an animation):

What do these plots tell us?

Well, for very close offsets (less than about 10 Hz), a good 10 MHz
oscillator can have much lower noise, even after multiplication in the
SDR-1000's DDS chip, than the internal 200 MHz oscillator.

From 10 Hz to about 10 kHz, a very good oscillator like the Wenzel can be a
bit better than the internal oscillator, while a more normal oscillator
like the Z3801A is slightly, but not dramatically, worse.

From 10 kHz on out, the noise generated by the DDS masks the difference
between different 10 MHz oscillators (except the Marconi, which is shown
as a worst-case example) and the internal oscillator can be as much as
10dB better.

But, there is something in the SDR-1000 that causes the phase noise, no
matter what the reference, to increase starting at about 30 kHz offset.
At 100 kHz offset, the internal reference only has a couple of dB
advantage over the external references, and even the Marconi performs as
well as the other references.

Although the difference is small, it's interesting to note that the
Z3801A outperforms the Wenzel ULN; this is odd since the Wenzel is known
to be better than the Z3801A. I suspect that different output levels from
the various oscillators might cause these small anomalies. Looks like it's
time for another experiment to determine how the reference signal level
impacts the noise...

Unfortunately, my test system only goes out to 100 kHz offset, so I don't
know what happens further out. But some of
John, K2OX's measurements show what appears to be a similar bump, and
his measurements, which go out to 1 MHz, indicate that the noise starts
going back down at about 300 kHz offset. It almost looks like there may
be another PLL involved, with a 30kHz bandwidth that operates whether or
not the DDS frequency multiplier is being used.